Salt bath furnaces



AF. HOLDEN SALT BATH FURNACES April 7, 1959 2 Sheets-Shes?.l 1

--Filed April 20. 1954 Fai..

lllllll Illlllll lll Il .4 7 Mm Z/H v INVENTOR Jfemas idf/aidez? /ZM April 7, 1959 A. F. HOLDEN SALT BATH FURNACES 2 Sheets-Sheet 2 Filed April 20. 1954 FIGB United States Patent O SALT BATH FURNACES Artemas F. Holden, Milford, Mich.

Application April 20, 1954, Serial No. 424,322

6 Claims. (Cl. 266-4) This invention relates to a nitriding process and to furnaces for the treatment of metals and more particularly to such furnaces in which a molten salt bath is used for heating the metal and in which a gas under pressure is introduced into the furnace in connection with the treatment process.

This application is a continuation-n part of patent application S.N. 711,047, of Artemas F. Holden, filed November 20, 1946, now abandoned.

In connection with the treatment of metals, particularly steel alloys, certain desirable properties of the alloys may be obtained by heating the alloys in a molten salt bath for a period vof time. During the heating period, a gas under pressure is sometimes introduced into the bath, as in nitriding, so as to aid in the treatment of the metal. The present invention is concerned with provision of a nitriding process and a furnace for this type of work.

It has been found that in such processes, and particularly in nitriding, it is essential to maintain a very constant temperature within the bath so as to carry out the process under controlled conditions, as well as to avoid warping or otherwise deforming the article being treated due to variations of temperature. It is likewise necessary for the same reasons to maintain a uniform temperature throughout the heating zone of the furnace regardless of the depth of the zone.

In accordance with the present invention, it is contemplated that the pot containing the molten salt bath in which the metal is treated is to be heated externally, and preferably by partial immersion in a second molten salt bath, in order to assure a constant and uniform temperature of the inner or working molten salt bath.

When a gas is bubbled through a molten salt bath,

there is danger of removing some of the salt from the furnace together with the gas escaping from the surface of the bath. If the gas isbeing conducted away through pipes as usual, there is-the attendant danger of the pipes becoming clogged by solidication upon. cooling of the molten salt carried-'intonthe pipes by the gas. This invention contemplatesvmaintaining a free space above the surface of the molten salt bath within the furnace and applying no heat -directly to this zone, a sucient drop in temperature occurring in thisv space so as to prevent the carrying of any molten salt bath from the furnace. A The invention further contemplates, in carrying out a nitriding process, the feeding of ammonia through a duct in heat-exchanging relationship with the outer salt bath and thence into the metal-treating salt bath, whereby the ammonia is preheated and dissociated prior to its entry into the treating bath. A catalyst-containing chamber forms a portion of such duct, the heated catalyst more thoroughly dissociating the ammonia than is possible by heat alone at'the temperature of the outer bath. The connection between the catalyst chamber and the remainder of theL duct is such that the salt of the treating bath within the inner pot cannot find its way into the catalystchamber even when the ammonia supply is shut off.

2,880,986 Patented Apr. 7, 19:59

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The gaseous atmosphere above the metal-treating bath is normally subjected to an appreciable, substantially constant pressure. In order to obtain even and uniform nitriding, the salt bath within the inner pot may be agitated at regular intervals as by allowing the gaseous atmosphere above the bath to escape through a time-controlled valve. Not only does such procedure periodically provide agitation of the bath, but also introduces large amounts of nascent nitrogen into the treating bath.

Among the objects of this invention is the provision of an improved nitriding process. A further object of this invention is to provide a furnace of the type described characterized by the uniformity of case hardening of the work pieces processed thereby and by its economy in the consumption of ammonia. Further objects of the invention reside in the provision of a furnace that will not be unduly chilled by the introduction of a cold charge of metal, that will maintain a substantially uniform temperature regardless of depth of the salt bath', which is durable, economical in construction and operation, and capable of being readily maintained and repaired.

A still further object of the invention resides in the provision, in combination with salt bath pressure nitriding equipment, of means for controlling the supply of ammonia to the furnace and for purging the spent gaseous products from above the treating bath, and to the method of operating such system.

My invention is clearly defined in the appended claims. In the claims, as well as in the description, parts are at times identified by specific names for clarity and convenience, but such nomenclature is to be understood as having the broadest meaning consistent with the context and with the concept of my invention as distinguished from the pertinent prior art. The best form in which I have contemplated applying by invention is illustrated in the accompanying drawings forming part of this specification, in which:

Fig. l is a vertical sectional view of the furnace taken along the line 1-1 of Fig. 3.

Fig. 2 is a vertical sectional view of the furnace taken along the line 2-2 of Fig. 3.

Fig. 3 is a view in top plan of the furnace, a cutaway portion of the furnace being shown in horizontal section taken along the line 3-3 of Fig. 2.

Referring to Figs. 1 and 3, a shell 10, preferably formed of heavy-gauge sheet steel, forms the outer structure of the furnace. The shell 10 may be reinforced at intervals by small metal angles 11 secured to the inside thereof by welding or otherwise in order to increase its strength. The shell 10 is preferably of rectangular cross section as shown in Fig. 3 and is lined with a layer of heat-insulating material 12 which is preferably of a refractory nature such as fire-brick in order to resist the relatively high temperature to which it is subjected.l

An outer container or pot 14 is positioned Within the insulating material 12 and is open at the top, the top flange 15 of the pot being at the level of the top edge of the shell 10. The pot 14 is formed of heavy-gauge sheet metal which is sufficiently resistant to corrosion by a molten salt bath at an elevated temperature, preferably of alloy steel such as chrome steel, stainless steel, or nickel-clad steel. The horizontal cross section of the pot 14 is preferably a composite semicircular and rectangular shape as shown in Fig. 3, which shape is the most convenient for the type of furnace herein disclosed. A layer of heat-resistant cement or similar material 16 may be used to seal the space between the flange 15 and the shell 10 above the insulating material 12 and about the pot 14 in order to protect the insulating material against damage and to improve the appearance of the A second or inner pot 17 is positioned within and spaced from the walls of the pot 14 and is adapted to con tain a second or working salt bath. The pot 17 is formed of heavy-gaugemetal suchas steelv alloy or, nickel-clad sheet steel which is resistant to corrosion of molten salt baths at elevated temperatures as well as to any effect of thenitriding or other metal treatment process to be car- .ried on withinV the pot. The pot 17 is preferably cylindrical in horizontal cross section and has a ilat bottom portion 19 which ris supported on a plurality of lugs 20 secured to the inside bottom of the pot 14 by welding or otherwise. The pot 17 also has an integral ange 21 overlying the flange of the pot 14 on three sides thereof so as to cover the major portion of the pot 14 as well as to aid in supporting the pot 17 as shown in Figs. 2 and 3. Lifting rings 22, 22 may be provided on the ilange 21 onfopposite sides of the pot 17 to aid in lifting the pot -for servicing or disassembly of the furnace.

The pot 17 is provided with a projecting hollow rib 24 which may be of generally U-shaped cross section, as shown in Fig. 3, and secured to the outer surface of the `potby welding. The rib 24 extends from the underside of ange 21 to substantially the bottom of the pot 17 as shown in Fig. 1. The rib 24 is closed at the top and bottom except for a connection leading laterally into it `adjacent the top thereof, to be described, and an opening 25 .formed in the wall of the pot 17 so as to connect the Vbottom of the pot with the bottom of the rib 24. A pyrometer 26 of the enclosed thermocouple type may be sealed into the interior passage of the rib 24 to extend to the ange 21 and for a portion of the distance down the pot. A vertical pipe 27 extends within the rib 24 to the opening 25, the bottom of the pipe being provided with an elbow and a short horizontal run 28 protruding and opening into the pot 17. The top of pipe 27, within the rib, is connected by means of an elbow to the short horizontal pipe 23 connected to the top of the catalyst chamber 33 to be described.

A cover 29 is provided for sealing the pot 17, the cover being formed of corrosion-resistant metal which may be similar to that from which the pot is formed. The cover 29 may be provided with stiffening ribs 30 to strengthen the cover and prevent distortion of the metal due to heating.V The cover 29 is provided with a downwardly projecting peripheral ridge 31 which cooperates With a peripheral groove 32 formed in the upper edge of the pot 17 to form a tightly fitting cover. An asbestos graphite gasket, not shown, positioned in the groove 32, may be used to form a gastight joint between the cover 29 and the pot 17 upon the cover being forced down tightly on the pot.

The flange 21 of the pot 17 preferably has a series of radially directed lugs 34 projecting upwardly therefrom at equal intervals `about the pot, as best shown in Fig. 3. Each of the lugs 34 has an opening through the upper porfion thereof. Pairs of spaced, radially disposed, cooperating lugs 36 are 'positioned on the top surface of the 'cover 39 to receivethe lugs 34 between them so that, when the cover is -inposition on the pot, the lugs 34 will lie between adjacent lugs 36.' The relative position of the lugs 34 and 36 is such that the cover 29 may be iirmly secured in position on the pot 17 by the insertion of wedges 37 into the openings in the lugs, the wedging action between the 'cooperating lugs forcing the cover 29 into firm ensuch pipe having' an `adjustable escape valve 61 therein.

A lifting ring`-45' is provided at the' central .portion lof the cover 29 to allow the cover to be removed from time to time in order to charge the furnace with metal to be treated or for servicing the furnace.

A series of electrodes 46are positioned across the rear side of the -pot 14, these electrodes being preferably formed of metal and of conventional L shape, one leg thereof projecting into the salt bath with which the pot 14 is intended to be filled.. The electrodes 46 are maintained in position by electrode clamps 47 which are supported on electrode brackets 49 secured vto the shell 10. Insulators 50 are positioned between the electrodes 46 and the electrode clamps l47. The rear side of the shell 10 and pot 14 may be somewhat lower than the other sides thereof to permit passage of the electrodes 46, it being desirable to maintain the tops of the electrodes below the top surface of the furnace. This construction of electrodes and supporting means is conventional and wellknown in the art. In the form shown, four electrodes are utilized for heating the salt bath within the pot 14,'these electrodes being connected to a suitable source of power by any of the usual circuits. It isobvious that the number of electrodes used for heating the salt bath may be varied without departing from the sc ope of this invention.

A pyrometer 51 of the incased `thermoeouple type is positioned Within the pot 14 in thermal contact with the salt bath therein. Both of the pyrometers 51 and 26 are connected to suitable conventional controls, not shown, for regulating the supply of current to the electrodes 41 so as to maintain the contents of either the pot 17 or the pot 14 not exceeding predetermined temperatures, the contents of the pot 14 being maintained at a predetermined temperature somewhat higher than that of the pot 17. The action of the controlslis toshut ot the power supply to the electrodes upon either salt` bath exceeding its predetermined temperature.`

Ammonia is supplied to the furnace through pipe 52, which extends through and is axed to the iiange 21, as shown in Fig. 2. A conventional shut-off valve 54 is interposed in the ammonia-supply pipe. Below the shut-olf valve the pipe, there designated 55, vlies vertically Within the vcatalyst chamber 33,- to which it is sealed atthe top, with the bottom end of the pipe spaced a short distance from the bottom of the chamber. Adjacent the top of the chamber, at a level above the top 57 of the salt bath 59 in the inner pot 17, is the short horizontal pipe 23 connecting the catalyst chamber to the top of pipe 27. The space in the catlyst chamber surrounding pipe 55 is substantially lled with an arnmonia-dissociating catalyst 56. The ammonia enters through pipe 52, passes downwardly through pipe 55, upwardly through the catalyst chamber 33, where it cornes into intimate contact with the catalyst and then, largely in the form of hydrogen and nascent nitrogen into which the ammonia has been dissociated, passes through pipe 23, downwardly through pipe 27, and thence into the bath 59 through the pipe 28.4

The catalyst is preferably in a 'granulated ory felted form lwhich does not materially obstruct the ow of gas therethrough. A preferred catalyst is granulated iron of a relatively large particle size. Other vcatalysts which may be used 4are wadded steel wool, which mayv -be packed in the chamber around the ,pipey 55. Still further catalysts which may be employed are granulated copper, nickel, chromium, and manganese.

In the operation of the furnace, the pot 14 is lled with a conventional salt bath composition Which is completely huid and has satisfactory characteristics at a temperature range from 900 to 1400 F. The salt vbath may be formed of combinations of nitrides, chlorides, or combinations of chlorides and carbonates, chlorides and fluorides, or chlorides, carbonates and iuorides.4 The inner or second pot is filled to approximately the same level as the outer pot, and preferably somewhat below that level, as indicated in Figs. l and 2. In the nitriding of steels, the inner salt bath 59 contains cyanide, preferably potassium, alcium, or sodium cyanide, and other salts to make up av bath-that .operatesat: the desired range of temperatures. The preferred bath is one made from the combination of potassium chloride to 30%; sodium cyanide 20% to 50%; sodium carbonate 15% to 3.0%; potassium, carbonate 15% to 30%; sodium fluoride 2% to 10%. The particular preferred bath used in the present case is a bath comprising potassium chloride sodium cyanide 36%, sodium carbonate 20%-, potassium carbonate. 19 and sodium fluoride 5%.

The pipe 52 is connected with a suitable supply of anhydrous ammonia supplied at a predetermined pressure, which is preferably a substantially constant. pressure and may be on the order of thirty pounds; per square inch. Appropriate control. valves, as disclosed in the United States Patent No.2,395,329, mayl be connected to the pipe. 52, the valves being adjusted to supply ammonia at a pressure of approximately thirty pounds per square inch. In addition, suitable equipment (not shown) for determining the ammonia content of the exhaust gases may be incorporated in the exhaust line, abovevalve 61, in accordance with the above-mentioned patent.

The pyrometers. 26 and 51 control the temperature of the furnaces by controlling. the supply of electric` power to the electrodes 46 in a well-known manner. When starting up the furnace, the controls may be set so as to operate the salt bath within the pot 14 at an initially higher temperature up to 1400 F. inorder to heat the furnace more rapidly. As they working salt bath within the inner pot 17 approaches the desired operatingtemperature which is preferably about 1000 F., the teinperature of the bath in the outer pot' 14 is dropped to the regular operating temperature of about 1050, F., this temperature' differential of approximately 50 Fiserving to maintain the bath within the inner pot 17 at a relatively constant operating temperature of about v1000" F. If the temperature of the bath in the outer pot'14 exceeds 1050 F. or the temperature 4of the bath in the inner pot 17 exceeds 1000 F., the power circuit to the electrodes is immediately interrupted.

As the inner pot 17 is charged with relatively cold pieces of steel to be nitrided, there will be a temporary drop in the temperature of the bath within this pot. This drop in temperature will be quickly overcome by the heat supplied from the bath within the outer pot 14. Moreover, inasmuch as the fluid bath within the outer pot 14 is free to circulate about the sides and the bottom of the pot 17, the inner pot 17 will be uniformly heated regardless of the depth of the pot. Hence, the outer pot 14 serves as a reservoir of heat to guard against sudden changes due to introduction of cold material and to apply heat uniformly to the inner pot 17.

It will be noted, from Figs. 1 and 2, that the top portion of the inner pot 17 is not directly heated by the salt bath within the pot 14. Further, the inner pot 17 is not completely filled, considerable space being left above the surface 57 of the bath so that very little, if any, of the molten bath 59 will be carried out of the pot into either of the exhaust lines 39 and 60. Further, to insure trouble-free operation, the valves 40 and 61 are placed substantial distances above the cover 29. 'Ihe top portion of the inner pot 17 is somewhat cooler than the bottom portion thereof due to the fact that it is not directly heated, as above mentioned, which also aids in the prevention of passage of the molten salt bath 59 with the exhaust gases to pipes 39 and 60.

The catalyst chamber 33 is substantially totally immersed in the outer salt bath 62 and thus is at substantially the same temperature as such bath. Consequently, the anhydrous ammonia supplied vthrough pipe 52 is quickly heated to that temperature, and, since it comes into intimate contact with the catalyst, it is very largely dissociated by the time it passes out through pipe 23. Pipe 27, in the rib 24, is likewise in thermal contact with -6 the bath 62, so that the gaseous products from the catalyst chamber remain at substantially the temperature of the outer bath until they pass out pipe 28 into the inner bath. Inasmuch as the temperature of the salt bath 62 is greater than that of 59, the gaseous products issuing from pipe 28 are at a higher temperature than such bath, and,l because of their initiald dissociation and the maintenance of them in that condition, they are particularly effective in the nitriding process. The pipe 23, through which communication is had between the catalyst chamber and the pipe27, is located above the top surface 57 of bath 59. As a'consequence, there is no possibility of the salt of bath 59 travelling up pipe 27 and through pipe 23 into the-catalyst chamber, even when the supplv of ammonia to pipe 52 is shut off.

Throughout the predominant part of the heating cycle, valve 40 is closed. Valve 61 is opened to such an extent as to maintain the gaseous products above bath 59 at a desired pressure and chemical analysis as indicated by the exhaust-gas-determining equipment (not shown) in the exhaust line. At predetermined intervals, preferably short, uniformly spaced intervals, the valve 40 is opened, thereby to purge the gases above the bath 59. The timer 42 may be set, for example, to open valve 40 for a period of fifteen seconds every three minutes. The partial release of pressure above the bath 59 by the opening of valve 40 causes agitation of the bath 59 and thus aids the thermal currents therein in the maintenance of substantially uniform temperature in the bath. The orifice through valve 40 is. of such area that it does not completelyrelease the pressure. in the inner pot, but causes the pressure therein` to dropappreciably when the valve is open. For example, when valve 40 is closed, the pressure abovethe inner bath will preferably be in the neighborhood of 30 p.s.i., and when valve 40 is opened such pressure will drop to about l5 psi., the ammonia entering through pipe 52 acting, tomaintain the pressure at such intermediate value.

1..A salt bath nitriding furnace comprising an outer pot adapted to contain a. first molten salt bath, electrodes positioned therein for heating the first salt bath, a second pot within the s-aid outer pot and adapted to be partially immersed in the rst saltbath, said second pot having a portion thereof exposed to the atmosphere outside of the outer pot and being adapted to contain a second, cyanide-containing molten salt bath heated by the first salt bath, means for sealing the inner pot, a fluid conduit hav ing a major portion thereof located within the first salt bath and heated by the rst salt bath to a temperature above that of the second salt bath for passing ammonia gas and the products of its dissociation into the lower portion of the second pot, said conduit passing in a first run downwardly into the first bath to a substantial depth therein, then rising in a second run to a level above the top of the second bath, and then passing downwardly in a third run to the lower portion of the inner pot, said conduit including as a part thereof before the third run a chamber containing a catalyst for aiding the dissociation of ammonia gas, and means for removing gas from above the bath in the pot, said last-named means including an exhaust conduit, a valve in the exhaust conduit, and agitating means for said second bath comprising means for automatically periodically opening the va ve.

2. A salt bath nitriding fru-ance comprising a pot adapted to contain a cyanidecontaining molten salt bath, means for sealing the pot, means for heating the pot, a fluid conduit heated by the heating means to a temperature above that of the salt bath for passing ammonia gas and the products of its dissociation into the lower portion of the pot, said conduit including as a part thereof a chamber containing a catalyst for aiding the dissociation of ammonia gas, and means for removing gas from above the bath in the pot, said last-named means including an 7 exhaust conduit, a valve in the exhaust conduit,- and agitating means for said second bath comprising means for automatically periodically opening the valve.

3. A salt bath nitriding furnace comprising an outer pot adapted to contain a first molten salt bath, means for heating the first salt bath, a secondpot Within the said outer pot and adapted to be partially immersed in the first salt bath, said second pot having 'a portion thereof exposed to the atmosphere outside of the 'outer pot and being adapted to contain a second, cyanide-containing molten salt bath heated by the first salt bath, means for sealing the inner pot, a fluid conduit having a major portion thereof located within the first salt bath and heated by the first salt bath to a temperature above that of the second salt bath for passing ammonia gas and the products of its dissociation into the lowerportion of the second pot, said conduit passing in a rst run into the first bath, then rising in a second run to a level above the top of the second bath, and then passing downwardly in a third run to the lower portion of the inner pot, said conduit including as a part thereof before the third run a chamber containing a comminuted catalyst for aiding the dissociation of ammonia gas.

4. A salt bath nitriding furnace comprising an outer pot adapted to contain a first molten salt bath, electrodes positioned therein for heating the first salt bath, a second pot within the said outer pot and adapted to be partially immersed in the iirst salt bath, said' second pot having a portion thereof exposed `to the atmosphere outside of the outer pot and being adapted to contain a second, cyanide-containing-molten salt bath heated by therst salt bath, means for sealing-the inner pot, a fluid conduit having a major portion thereof'located within the iirst salt bath and heated by the first-salt bath to a temperature above that of the second saltbath for passing ammonia gas and the'products of its dissociation into the lower portion of the second pot, said conduit passing in a iirst run downwardly into the first bath to a substantial depth therein, then rising in a second run to a level above the top of the second bath, and then passing downwardly in a third runto the lower portion of the inner pot, said conduit including as a part thereof before the third run a chamber containing a comminuted catalyst for aiding the dissociation of ammonia'gas.

5. A salt bath furnace for the nitriding of metals, com'- 'prising a pot ladapted tofcontain'a cyanide-containing metal-treating bath, a uid conduit for introducing ammonia gas into said salt bath at the lower portion of said pot, a cover for the pot for sealing such pot, and means for removing gas from above the bath in the pot, saidv last-named means including angexhaust conduit, a valve connected to theexhaust-conduit, and agitating means for said second bath comprising means for automatically periodically opening the valve.

6. A salt bath nitriding furnace comprising an outer pot adapted to contain a first molten salt bath, means for heatingA the first salt bath, a second pot within the said outer pot and adapted to be partially immersed in the first salt bath, said second pot having a portion thereof exposed tothe atmosphere outside of the outer pot and being adapted to contain a second, cyanide-containing molten salt bath heated by the rst salt bath, means for sealing the inner pot, a uid conduit for introducing ammonia gas into said salt bath at the lower portion of said pot, a cover for the pot for sealing such pot, and means for removing gas from above the bath in the pot, saidrlast-named means including an exhaust conduit, a valve in the exhaust conduit, and agitating means for said second bath comprising means for automatically periodically opening the valve.

References Cited in the tile of this patent UNITED STATES PATENTS 1,907,331; MilliE May 2, 1933 f 1,941,128 Dul Val Dec. 26, 1933 1,999,757W VI-Iarsch Apr. 30, 1935 2,041,769 Larkin May 26, 1936 'v '2,284,051 v Johnsonl May 26, 1942 i 'v2 ',1 a"9,5,329 Holden Feb. 19, 1946 2,552,260 `Cooper May 8, 1951 2,630,315 Munford Mar. 3, 1953 :22,663,379y Doan Dec. 22, 1953 FOREIGN PATENTS 514,826 Y France Nov. 19, 1920 701,149 France Jan. 5, 1931 848,340 France July 24, 1939 

2. A SALT BATH NITRIDING FRUANCE COMPRISING A POT ADAPTED TO CONTAIN A CYANIDE-CONTAINING MOLTEN SALTS BATH, MEANS FOR SEALING THE POT, MEANS FOR HEATING THE POT, A FLUID CONDUIT HEATED BY THE HEATING MEANS TO A TEMPERATURE ABOVE THAT OF THE SALT BATH FOR PASSING AMMONIA GAS AND THE PRODUCTS OF ITS DISSOCIATION INTO THE LOWER PORTION OF THE POT, SAID CONDUIT INCLUDING AS A PART THEREOF A CHAMBER CONTAINING A CATALYST FOR AIDING THE DISSOCIATION OF AMMONIA GAS, AND MEANS FOR REMOVING GAS FROM ABOVE THE BATH IN THE POT, SAID LAST-NAMED MEANS INCLUDING AN EXHAUST CONDUIT, A VALVE IN THE EXHAUST CONDUIT, AND AGITATING MEANS FOR SAID SECOND BATH COMPRISING MEANS FOR AUTOMATICALLY PERIODICALLY OPENING THE VALVE. 